The term "lead chromate pigment" as used herein refers to the pigments identified as Chrome Yellow CI-77600 Lead Sulfochromate CI-77603, and Molybdate Orange CI-77605 in the "Colour Index", second edition, 1956, published jointly by the Society of Dyers and Colourists, England, and the American Association of Textile Chemists and Colorists, United States. The yellow lead chromate pigments include a very green shade "Primrose Yellow" in rhombic crystal form, a relatively pure lead chromate in monoclinic crystal form commonly known as "Medium Yellow" and solid solutions of lead chromate and lead sulfate in monoclinic form known as "Light Yellows." The molybdate oranges vary in color from a relatively light masstone, strong and yellow tint (small particle size) to a dark, red masstone and weak, red tint (relatively large particle size).
Lead chromate pigments have been prepared by a variety of methods, most of which involve precipitation of the lead chromate pigment from aqueous solutions of its constituent ions, i.e., lead, chromate, sulfate and for molybdate oranges, molybdate, in amounts varying according to the shade of pigment desired. Conventionally, an aqueous solution containing the soluble anionic salts is mixed with a lead salt, in the form of an aqueous slurry or an aqueous solution, depending on the solubility of the lead salt. After precipitation, but prior to isolation, the lead chromate pigment is commonly treated with a variety of hydrous oxides among which are hydrous oxides of aluminum, titanium, manganese, silicon, antimony and bismuth or mixtures thereof to provide a loose porous coating on the surface of the pigment to enhance pigmentary properties, such as light-fastness in paint compositions, as described, for example, in U.S. Pat. Nos. 2,212,917 2,808,339, and 2,813,039. The red shade molybdate orange pigments are conventionally further treated with an antimony compound, commonly antimony trichloride, prior to isolation.
The major contribution toward color of lead chromate pigment resides in the lead chromate component. Likewise, many of their shortcomings are also attributable to the properties inherent in the lead chromate component. Among these are (1) sensitivity to alkalies and acids, (2) staining in the presence of sulfides and (3) darkening on exposure to light or to elevated temperatures.
Early attempts to overcome these deficiencies have usually involved special treatment of the pigments designed either to provide a means of neutralizing the attacking agent and rendering it temporarily ineffective or to provide a barrier against the ready access thereof to the sensitive pigment particle. The claimed improvements, although very frequently demonstrable, have usually been only of academic interest in that their effectiveness has been short lived and not of sufficient duration to render possible the use of these pigments in many applications, particularly those applications where a high degree of thermal stability is required.
More recent and relatively successful attempts to overcome these deficiencies are described, for example, in U.S. Pat. Nos. 3,370,971 and 3,639,133. These patents describe coated lead chromate pigments, which are chemically resistant, lightfast and relatively thermally stable. The pigments therein described are prepared by applying a coating of dense, amorphous silica and optionally alumina to lead chromate pigment which has been precipitated and after-treated with hydrous oxide in a conventional manner. The dense silica-coated lead chromate pigments are described generally as resistant to darkening when heated to 300.degree.to 320.degree. C. in such plastics as polyethylene or polystyrene. A dense silica-coated molybdate orange pigment is described in Example 1 of U.S. Pat. No. 3,370,391 as exhibiting no significant discoloration up to about 250.degree. C. and minor discoloration up to 315.degree. C. in a thermoplastic resin, compared to a conventional molybdate orange which darkens noticeably at 230.degree. C. A dense silica-coated Medium Chrome yellow is described in Example 7 of that patent as showing only small color loss at 288.degree. C. as compared to a conventional medium chrome yellow.
Although the dense silica-coated lead chromate pigments do show significant improvement in thermal stability when compared with uncoated pigment and gel-coated pigment known in the art, the resistance to darkening exhibited by these pigments at 300.degree.-320.degree. C. is low enough under many processing conditions to necessitate the use of temperatures from 30.degree.-60.degree. C. lower than the stated range to insure adequate color integrity. The resistance to darkening is further decreased in proportion to the length of time the pigment is exposed to elevated temperatures. For example, a pigment which retains color integrity when initially reaching an elevated temperature can darken considerably after being maintained at that temperature for several minutes.
The need to avoid elevated temperatures and extended exposure to elevated temperatures has a particularly adverse impact on the thermoplastics industry. In the thermoplastics industry pigmented thermoplastics are often held at the extrusion temperature for as long as 30 minutes prior to extrusion, during which time the pigment can significantly darken. In addition, the speed of further processing depends primarily on high temperature of the thermoplastic. In the processing of thermoplastics the use of temperatures even 30.degree. C. below 300.degree. C., not to mention 60.degree. C. below, can decrease polymer flow and increase residence time of the polymer in the mold to the extent that process efficiency suffers and overall productivity decreases.
This invention provides for silica-treated lead chromate pigment which, when coated with dense, amorphous silica, retains a significantly higher degree of color integrity at high temperatures than conventional dense silica-coated lead chromate pigments, while retaining and, in some cases surpassing, the degree of chemical resistance and lightfastness exhibited by the conventional lead chromate pigments.